Thursday, October 14, 2010

Although climate predictions indicated an unusually hot and dry summer for 2010, tropical storms turned out to be the dominant influence on the weather of Central Texas accounting for nearly half of the annual rainfall to date (Figure 1).Just as springflow and groundwater levels had reached their peak in late spring and were beginning their natural seasonal decline, Tropical Storm Alex struck in June and brought above-average rainfall for the month.Alex originated in the Caribbean and was the first tropical cyclone to form in the 2010 Atlantic Hurricane season; it made landfall just south of Texas and proceeded into inland Mexico and then into Texas.The storm brought up to 6 inches of rain in Central Texas.Alex was followed by a tropical depression in July that produced a couple of inches of rainfall.August turned out to be one of the hottest and driest months on record, but then in September Tropical Storm Hermine produced up to 10 inches of rain, which is turning out to be one of the wettest on record for the region.These three, well-spaced tropical systems helped provide significant rainfall and creekflow, resulting in rises in groundwater levels and springflow for the Barton Springs segment of the Edwards Aquifer (Figure 2).

Tropical Storm Hermine

The most significant rainfall totals were associated with Tropical Storm Hermine. This storm originated off the coast of Mexico and quickly strengthened before making landfall in Mexico just south of Texas on September 7, 2010.Hermine remained a well-organized storm system and brought a lot of rain and flooding to Texas as the eye of the storm finally dissipated in Central Texas.As a result, Central Texas received a remarkable amount of rainfall, (Figure 3) with 15.6 inches reported from Georgetown and 11.5 inches reported for Austin.Locally the District recorded about 10 inches (Figure 1).

Significant rainfall from Hermine generated runoff and caused flooding in both the contributing and recharge zones of the Edwards Aquifer (Figures 4 and 5).Peak streamflow in the creeks was orders of magnitude greater than rises caused by Hurricane Alex, and the duration of flow also much greater for Hermine than Alex.Enough runoff was generated to sustain flow in the larger watersheds (Blanco and Onion) for several weeks; the Blanco River continues to flow across the entire recharge zone.However, most creeks that contribute recharge to the Barton Springs aquifer are now only flowing at the upstream portion of the recharge zone and are declining rapidly.However, despite having significantly more water flowing in the creeks, water levels in the aquifer did not rise above their early summer levels (Figure 2) and are currently leveling off or declining.Barton Springs flow reached its highest sustained discharge following Hermine, but it is currently only about 10 cfs higher than early summer flows (Figure 2).

The recharge that occurred will sustain water levels for the next few months, and has effectively reset and delayed the starting point of the natural decline of groundwater levels and springflow that had initially started in August.

The nature of this karst aquifer is to respond to climatic events and stresses quickly, and indeed the response to a hot August followed by a wet September illustrates that fact.This article was written one month after the rains of Hermine passed, and the longer-term impacts of Hermine will be more apparent in future months.

Outlook

Although the short-term (6-month) outlook for the region is very good due to the higher-than-average springflow and water levels, we need to be mindful that conditions will change.The Climate Prediction Center (CPC) has issued a “La Niña Advisory.”The CPC states that La Niña conditions are expected to last through winter 2010-2011.The El Niño-La Niña cycles result from changes in the global moisture patterns related to ocean temperatures fluctuations in the equatorial Pacific Ocean.La Niña conditions usually produce drier-than-normal conditions for Central Texas.

Figure 1. Monthly rainfall chart from the District office in Manchaca, South Austin.Note the dual peaks in rainfall in 2010 associated with Hurricane Alex and Tropical Storm Hermine.The rainfall in July came from a tropical depression that kept the total for the month near the historical average; without that tropical depression, July would have been drier than normal, similar to August.

Figure 2. Hydrograph of water levels, streamflow, and Barton Springs for the Summer of 2010. Streamflow and springflow data from USGS (Blanco River at Wimberley, Onion Creek at Driftwood, and Slaughter Creek at FM 1826). Water-level data from the BSEACD (Porter well is located near Buda, and the Lovelady well is the District’s drought trigger well located in South Austin).

Figure 4. Many low-water crossings were flooded by the rainfall from Hermine. This photo is of a low-water crossing of Slaughter Creek just west of Brodie Lane. You can see the high-water mark on the road near the top of the photo.Photo taken September 8, 10:00 am.

Figure 5. Flooding in Barton Creek brought floodwaters into Barton Springs Pool.This photo was taken September 8, 2010 at about 12:00 pm.

Drought Rules

Either Barton Springs OR the Lovelady Well can trigger a drought declaration by the Board. However, BOTH Barton Springs AND the Lovelady Well must be above their respective drought trigger levels to exit a drought declaration.